1. Field of the Invention
The present invention relates to a turbocharging system for an internal combustion system equipped with an internal combustion engine and a plurality of turbochargers.
2. Description of the Related Art
As a conventional turbocharging system for an internal combustion engine equipped with an internal combustion engine and a plurality of turbochargers, there is a two-stage turbocharging system having a small size turbocharger on a high-pressure stage, and a large size turbocharger on a low-pressure stage. For instance, Patent Documents 1 and Patent Document 4 disclose a two-stage turbocharging system of this type.
An example structure of the conventional two-stage turbocharging system having the large size turbocharger and the small size turbocharger is explained in reference to FIG. 8 and FIG. 12. FIG. 8 illustrates an example of the conventional two-stage turbocharging system. As shown in the drawing, the conventional two-stage turbocharging system comprises an internal combustion engine 1, and two turbochargers 2A and 2B driven by exhaust gas from the internal combustion engine. The two-stage turbocharging system further comprises three control valves V1 to V3 for switching flow paths of intake gas drawn into the internal combustion engine 1 and flow paths of the exhaust gas from the internal combustion engine 1, and a control unit 3 which controls the control valves V1 to V3 and the turbocharger 2A and 2B. Herein, the control valves V1 to V3 respectively function as a bypass valve, flow control valve and a wastegate valve.
Further, the two-stage turbocharging system further comprises an intercooler 5 which is arranged on an upstream side of the internal combustion engine 1 (in an intake path) and cools the air having been heated by compression by the turbochargers while maintaining the pressure of the air. Furthermore, the two-stage turbocharging system comprises an air cleaner 4A which is arranged on the compressor side of the turbocharger 2B, and a muffler 4B. The air cleaned in the air cleaner 4A is fed to the turbocharger 2B and the exhaust gas from the turbochargers 2A and 2B is introduced to the muffler 4B.
Now, control patterns with use of the two-stage turbocharging system of the above configuration are explained hereinafter.
FIG. 9 illustrates a gas flow in a complete two-stage turbocharging state in which the rotation speed of the engine is approximately 1000 to 1250 rpm. In this state, all the control valves V1 to V3 are closed. And the intake air is introduced through the compressor side of the turbochargers 2A and 2B respectively and then fed into the intercooler 5. Next, the intake air is introduced into the internal combustion engine 1. The exhaust gas from the internal combustion engine 1 is discharged to outside of the system after passing through the turbine side of the turbochargers 2A and 2B respectively and the muffler 4B. In FIG. 9, the turbocharger 2A and the turbocharger 2B are connected in series and the turbocharger 2B is bigger than the turbocharger 2A.
FIG. 10 illustrates a gas flow in a variable two-stage turbocharging state in which the rotation speed of the engine is approximately 1250 to 2500 pm. In this state, the control valves V1 and V3 are closed and the control valve V2 is half-open. The control valve V2 is a flow control valve and thus the flow of the exhaust gas can be adequately controlled by changing the opening of the valve. With use of the control valve V2, the flow rate of the exhaust gas to the turbocharger 2A can be controlled, thereby controlling the turbine output. And the exhaust gas through the turbocharger 2A and the exhaust gas through the control valve V2 flow into one before entering the turbocharger 2B. The exhaust gas introduced to the turbocharger 2B is discharged to outside via the muffler 4B.
FIG. 11 illustrates a gas flow in such a state that the rotation speed of the engine is approximately 2500 to 3500 pm. In this state, the control valve V3 is closed and the control valves V1 and V2 are open. In this case, the flow paths having the control valves V1 and V2 have a larger cross-sectional area than the flow paths going through the turbocharger 2A, and thus a majority of the air and the exhaust gas flow through the flow paths having the control valves V1 and V2. As a result, the turbocharger 2A is not operated much and is in an idling state.
FIG. 12 illustrates a gas flow in such a state that the rotation speed of the engine is approximately 3500 rmp or above. In this state, the control valves V1 and V2 are full-open and the control valve V3 is half-open. The control valve V3 is controlled so as to control the flow rate of the exhaust gas to the turbine of the turbocharger 2B, thereby controlling the output of the turbocharger 2B.
As shown through FIG. 9 to FIG. 12, the internal combustion engine 1 is in the two-stage turbocharging state at a low/medium speed (1000 to 2500 rpm), and is in the one-stage turbocharging state at high speed (2500 rpm and above). In the one-stage turbocharging state, only the turbocharger 2B (low pressure turbo) is activated. Alternatively, only the turbocharger 2A (high pressure turbo) may be used instead of the turbocharger 2B in the single-state turbocharging state. As a result, this turbocharging system has a lot of flexibility to meet load change.
Moreover, the two-stage turbocharging system disclosed in Patent Document 4 operates both the turbines in series in a low speed/low load operation, and operates the turbines in parallel in a high speed/high load operation.